The invention concerns manufacturing of propylene derivatives. More specifically, the invention concerns the selective purging of propane and recovery of propylene in the process by using gas separation membranes to treat the reactor vent stream.
The United States produces more than 10 billion pounds annually of chemicals derived from propylene. Important derivatives include acrylonitrile, butyl alcohol, propylene oxide, isopropyl alcohol and cumene.
In a typical propylene derivative manufacturing process, propylene and other reagents are introduced into a high-pressure reactor. The raw effluent from the reactor is transferred continuously to one or more separation steps, from which a stream of raw derivative product is withdrawn for further purification. A stream of overhead gases, containing unreacted propylene, is also withdrawn from the separation steps. If the conversion of propylene to product is high, such as 95% or above, these overhead light gases may simply be sent to the fuel line. In many cases, however, propylene conversion per pass is much lower than this, and the overhead gas is recirculated to the reactor. Thus, the propylene feed to the reactor is a combination of fresh propylene and propylene recirculated in the reactor/product separation process loop. The fresh feed is usually chemical-grade propylene, a high-purity reagent that has a propylene content of about 95% or above, the remaining 5% or less being mostly propane, which passes unchanged through the reactor. Although the proportion of inert gas introduced into the reactor loop with the fresh feed in this way is small, the amount circulating builds up quickly, reducing the catalyst activity and reactor productivity. Propane build-up is usually controlled to a steady-state propane content in the loop in the range about 5-30%, by continuously venting a small fraction of the overhead gas from the recirculation loop.
Such a purge operation is unselective however, and, since the vent stream may contain as much as 90 vol % or more of propylene, multiple volumes of propylene may be lost from the loop for every volume of propane that is purged. Even though the volume of gas vented is only a few percent of the volume of fresh feed, the propylene lost in this way may typically amount to a few million pounds per plant per year, with a value of $1 million or more.
Despite its high value, propylene recovery from the purge stream, by separating it from the propane before the propane is vented, is generally not cost effective. Separation of propylene from propane is difficult, because of the similar physical properties, including close boiling points (propylene, xe2x88x9248xc2x0 C. and propane xe2x88x9242.2xc2x0 C.). When high-purity propylene is manufactured, it is separated from propane in a C3 splitter, a large cryogenic distillation column that typically contains 150 or more trays. It is clearly not practical or economic to install such equipment solely for purge treatment. Pressure swing adsorption (PSA), which can make product streams of high purity, has also been considered, but available adsorbents are not very effective, and PSA systems are also costly and energy intensive.
In summary, the concentration of propane in the reactor is regulated by the rate of purging via the vent stream; to achieve maximum productivity from the reactor and a high conversion of propylene to product, the propylene concentration in the reactor should be as high, and the propane content as low, as possible. Without the ability to recover propylene from the purge gas, however, there is an inevitable trade-off between controlling propane concentration in the reactor and losing propylene feedstock in the purge vent stream, by which operators of polypropylene plants are constrained. By purging a chosen percentage of the effluent light overhead gas, the operator makes what is for him, in the circumstances specific to the plant, the most acceptable compromise between the two undesirable extremes of excessive propylene loss and excessive loss of reactor efficiency.
Separation of propylene from propane by means of membranes is discussed extensively in the literature. It is well known that numerous materials and membranes exist with intrinsic selectivity for propylene over propane. These include facilitated transport membranes, polymeric membranes and inorganic membranes.
Membranes are not immediately attractive, however, for propylene recovery from vent gas, because, unlike PSA and cryogenic distillation, membranes are not able to produce a high-purity propylene permeate stream and at the same time achieve high levels of propylene recovery. The reason for this is that a membrane is not a perfectly selective barrier. If the membrane area and time in which the molecules of the gas stream are in contact with the membrane surface are very small, only a very small cut of the total feed flow will permeate. Since propylene permeates faster than propane, most of this small permeate cut will be propylene. That is, the permeate stream will have high propylene purity, and the residue stream will have a composition that is not much changed from the membrane feed composition. In other words, most of the propylene that was present in the feed gas will remain on the feed side, and will be lost when that gas is vented. Propylene recovery can be increased by increasing membrane area and contact time for the gas molecules. However, in this case propane permeation will also be increased. In other words, increasing propylene recovery also results in increasing propane recovery. Thus, little propylene will remain on the feed side to be lost by venting, but the recovered gas will be of low propylene purity.
Furthermore, the propylene/propane selectivity that can be obtained from most membranes under real operating conditions is small. Although literature references cite propylene/propane selectivities even as high as 50 or more, these data have generally been obtained from experiments with pure gases under low feed pressure conditions and with a vacuum on the permeate side of the membrane. With gas mixtures at high pressures, the best propylene/propane selectivity that can be obtained is typically no higher than between 2 and 3.
Despite these inherent difficulties, it has been proposed to apply membrane separation to the recovery of light olefins from reactor vents. U.S. Pat. No. 4,623,704 describes such a process for recovering ethylene from the reactor vent of a polyethylene plant. In this case of polyethylene manufacturing, the reactor is run at very high ethylene, very low ethane levels, so that the vent stream contains 96.5% ethylene, only 2.7% ethane and smaller amounts of methane and nitrogen. The stream is passed across a cellulose triacetate membrane that is selective for ethylene over ethane. Although the membrane selectivity is poor, the membrane produces an upgraded permeate stream, now containing 97.9% ethylene and 1.5% ethane, which is considered sufficiently free of impurities for return to the reactor, and a residue stream containing 89.9% ethylene, .8.5% ethane, which is purged from the reactor loop and used as fuel gas.
A chapter by R. D. Hughes et al., entitled xe2x80x9cOlefin Separation by Facilitated Transport Membranesxe2x80x9d, in Recent Developments in Separation Science, N. N. Li et al. (Eds), CRC Press, 1986, discusses pilot-scale tests of a facilitated-transport membrane module at a polypropylene plant. The module was used to treat vent gas from the reactor with a view to recovering propylene. The test was a technical success for the membranes, in that the module was able to produce a permeate stream typically containing about 97-99% propylene. However, since the membrane process could not produce polymer-grade propylene, the permeate was not recirculated to the reactor, and the process was not pursued.
Thus, recovery of propylene from the propane vent stream of reactors using propylene as a feedstock has been recognized to be desirable for many years. It has also been recognized that the recovered propylene needs to be of comparably high purity to the fresh reactor feedstock if it is to be recirculated in the process. Although methods for separating propylene from a propylene/propane mixture exist, they are impractical for vent stream treatment, either because they are too costly, or because they cannot produce propylene of sufficient purity.
The invention is an improved process for making propylene derivatives, specifically, but not exclusively: cumene; chlorohydrin, a precursor of propylene oxide; isopropyl alcohol; and butyraldehyde, a precursor of butyl alcohol. The process involves carrying out a reaction of propylene with the appropriate reagent or reagents in a reactor, separating the derivative product from the reactor effluent, and recirculating unreacted propylene to the reactor. The process includes a membrane separation step to provide selective purging of propane and other inert components from the reactor loop, and to recover a propylene-enriched stream for return to the reactor. In a basic aspect, the process of the invention comprises the following steps:
(a) carrying out one of the following reaction steps in a reaction zone:
(i) the reaction of propylene with hypochlorous acid wherein the propylene derivative chlorohydrin is made;
(ii) the reaction of propylene with water wherein the propylene derivative isopropyl alcohol is made;
(iii) the reaction of propylene with carbon monoxide and hydrogen wherein the propylene derivative butyraldehyde is made;
(iv) the reaction of propylene with benzene wherein the propylene derivative cumene is made;
(b) withdrawing from the reaction zone an effluent comprising propylene, propane and the propylene derivative;
(c) subjecting the effluent to at least one separation step, thereby producing a raw propylene derivative stream and a gas stream;
(d) passing at least a portion of the gas stream across a feed side of a membrane selective for propylene over propane;
(e) withdrawing from a permeate side of the membrane a permeate stream enriched in propylene compared to the gas stream;
(f) withdrawing from the feed side a residue stream enriched in propane compared to the gas stream;
(g) recirculating at least a portion of the permeate stream to the reaction zone.
The reaction and separation steps (a) through (c) are carried out as generally taught in the prior art. The reaction itself may take place in the gas phase or the liquid phase, using such reagents, catalysts, solvents and other additives as are known and available. Specific reagents and reaction schemes to make the particular propylene derivatives are discussed in more detail below.
The separation step that creates the crude propylene derivative product stream and the propylene-containing gas stream may be carried out in any convenient manner, such as by flashing, cooling/condensation, distillation, absorption or combinations of these, depending on whether the effluent from the reactor is in the liquid phase or the gas phase, and on what other components are present.
The membrane separation steps (d) through (f) may be carried out on the entirety of the stream to be recirculated to the reactor, or on a part of the stream, with the other part of the stream being recirculated directly to the reactor. In processes where the propylene conversion per pass is high and the propylene concentration in the fresh feed is high, the volume flow rate of overhead gas to be treated is comparatively small, and it is often cost-effective to treat the entire stream. In processes where the propylene conversion per pass is relatively low, and/or the propylene concentration in the fresh feed is low, the volume flow rate of overhead gas to be treated is comparatively large, and it will frequently be impractical to treat more than a portion of the stream.
The membrane separation steps may take the form of a single membrane separation operation or of multiple sub-operations, depending on the feed composition, membrane properties and desired results.
The membrane feed stream typically contains more than 5% propane and less than 95% propylene. The membrane separation steps produce a residue stream with a relatively high concentration of propane, such as as much as 30%, 40% or more, which is vented from the reactor loop. In this way, the amount of propylene vented from the process is reduced, compared with prior art unselective purging. Typically, the amount of propylene lost with the vented propane may be reduced from, for example, four volumes of propylene per volume of propane to one or two volumes of propylene per volume of propane. The amount of propylene removed can be controlled by varying the stage-cut at which the membrane unit operates, as explained in the Detailed Description below.
The membrane separation steps also produce a permeate stream enriched in propylene compared with the membrane feed stream, but not of high propylene purity. The higher the propylene recovery, the less enriched in propylene will be the permeate stream. Thus, the membrane permeate stream typically has a propylene concentration that is below 95% or below 90%. This stream is recirculated directly or indirectly to the reaction zone.
As mentioned, the membrane permeate stream usually comprises only part of the recirculated gas, the other part being gas that is recirculated directly in the loop without passing through membrane treatment. However, the propylene content of the recirculated membrane permeate stream is increased at least slightly compared with the untreated gas. Therefore, when the present process is compared with processes in which none of the recirculated gas has been treated by membrane separation, the recirculated gas has a slightly higher propylene concentration than in the comparative case. Thus, the process can provide a slightly higher propylene partial pressure and correspondingly lower propane partial pressure in the reactor than was achieved previously. This is beneficial in increasing catalyst life and efficient use of reactor capacity.
Additional separation steps may be carried out in the loop as desired to supplement the crude product separation or membrane separation steps or to remove secondary components from the stream.
The process of the invention may also be found to be useful from time to time for the manufacture of other propylene derivatives than those specified above, by following essentially the same set of process steps, namely reaction, separation of raw product from light gases, treatment of the light gases to separate propylene from propane, and recirculation of the recovered, but not highly purified, propylene to the reaction zone.
In another aspect, the invention is reactor apparatus comprising a reactor loop incorporating the reactor itself, the product separation equipment and the membrane separation unit containing a propylene-selective membrane.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.